[PATCH v3 00/13] mm/huge_memory: refactor zap_huge_pmd()

Lorenzo Stoakes (Oracle)
2026-03-20T18:14:59Z

The zap_huge_pmd() function is overly complicated, clean it up and also add
an assert in the case that we encounter a buggy PMD entry that doesn't
match expectations.

This is motivated by a bug discovered [0] where the PMD entry was none of:

* A non-DAX, PFN or mixed map.
* The huge zero folio
* A present PMD entry
* A softleaf entry

In zap_huge_pmd(), but due to the bug we manged to reach this code.

It is useful to explicitly call this out rather than have an arbitrary NULL
pointer dereference happen, which also improves understanding of what's
going on.

The series goes further to make use of vm_normal_folio_pmd() rather than
implementing custom logic for retrieving the folio, and extends softleaf
functionality to provide and use an equivalent softleaf function.

[0]:https://lore.kernel.org/all/6b3d7ad7-49e1-407a-903d-3103704160d8@lucifer.local/

v3:
* Propagated tags, thanks everybody!
* Fixed const vma parameter in vma_is_special_huge() in 1/13 as per
Sashiko.
* Renamed needs_deposit -> has_deposit as per Kiryl, better describing the
situation as we're zapping deposited tables, not depositing them.
* Initialised has_deposit to arch_needs_pgtable_deposit(), and updated huge
zero page case to account for that as per Kiryl.
* Dropped separated logic approach as per Baolin.
* Added 'No functional change intended.' caveats.
* Removed seemingly superfluous, inconsistent pot-folio_remove_rmap_pmd()
mapcount sanity checks.
* De-duplicated tlb->mm's.
* Separated folio-specific logic into another function.
* Added softleaf_is_valid_pmd_entry(), pmd_to_softleaf_folio() functions.
* Add and use normal_or_softleaf_folio_pmd() to make use of
vm_normal_folio_pmd() and pmd_to_softleaf_folio() for obtaining the
folio.
* Add and use has_deposited_pgtable() to figure out deposits.
* Added a bunch of explanatory comments as per Baolin.

v2:
* Added tags thanks everybody!
* Fixed issue with returning false on bug case potentially looping forever as
per Baolin.
* Fixed further issue in bug path in 5/8 with double pte unlock.
* Add patch to use vm_normal_folio_pmd() as per David.
https://lore.kernel.org/all/cover.1773924928.git.ljs@kernel.org/
v1:
https://lore.kernel.org/all/cover.1773865827.git.ljs@kernel.org/

Lorenzo Stoakes (Oracle) (13):
mm/huge_memory: simplify vma_is_specal_huge()
mm/huge: avoid big else branch in zap_huge_pmd()
mm/huge_memory: have zap_huge_pmd return a boolean, add kdoc
mm/huge_memory: handle buggy PMD entry in zap_huge_pmd()
mm/huge_memory: add a common exit path to zap_huge_pmd()
mm/huge_memory: remove unnecessary VM_BUG_ON_PAGE()
mm/huge_memory: deduplicate zap deposited table call
mm/huge_memory: remove unnecessary sanity checks
mm/huge_memory: use mm instead of tlb->mm
mm/huge_memory: separate out the folio part of zap_huge_pmd()
mm: add softleaf_is_valid_pmd_entry(), pmd_to_softleaf_folio()
mm/huge_memory: add and use normal_or_softleaf_folio_pmd()
mm/huge_memory: add and use has_deposited_pgtable()

include/linux/huge_mm.h | 8 +-
include/linux/leafops.h | 39 +++++++++-
include/linux/mm.h | 16 ----
mm/huge_memory.c | 168 +++++++++++++++++++++++++---------------
4 files changed, 143 insertions(+), 88 deletions(-)

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2.53.0